The present invention relates to a bend limiting device for cables and in particular to a bend limiting device for submarine cables. Both in the case of an electric cable and in the case of an optical cable, the greatest risks of damage due to bending exist along discontinuous portions of the cable itself, in particular in the region of rigid joining elements. In fact, in the case of bending of the cable, and in particular in the case of bending during traction, since the cable has a relatively poor flexural strength, the curvature of the cable portion leaving the joining element may have a very small radius, which may be such as to result in damage or breakage of the cable itself. Basically, each type of cable has its own limit value for the radius of curvature, below which the functional capacity of the cable itself is adversely affected.
In order to reduce the risks of breakage of a cable in the region of the joining elements, bend limiting devices are generally used. These usually comprise a flexible elongated body to be applied on the outside of the cable in the region of the joining element and able to provide a predefined flexural strength, ensuring that the curvature assumed by the cable itself does not reach critical values. For example, the patent U.S. Pat. No. 5,461,690, in the name of ATandT IPM Corp., proposes an apparatus for limiting the bending of a cable to be used in the region of a connector. The apparatus has an elongated cylindrical shape and is composed of elastic material able to withstand loads within predefined limits. On its external surface, the apparatus has circumferential grooves which delimit one or more weakened portions which are designed to bend under the action of relatively small loads. This apparatus is suitable for cables with small dimensions, for example cables with a single optical fibre, and the loads which can be withstood by the apparatus itself are those which may occur during use of cables of this type.
In some applications, however, the loads to be withstood are very high, for example several tonnes. Loads of this order of magnitude occur for example during the process of laying of submarine cables. Submarine cables are electric or optical cables used for conveying energy or for long-distance telecommunications and designed to be laid on sea or ocean beds. The diameter of these cables may vary between about 14 mm and 50 mm in the case of optical cables and between about 50 mm and 160 mm in the case of energy supply cables.
Submarine cables generally comprise sections of cables of limited length which are joined by rigid joining elements. In the case of telecommunications cables, the lengths of the sections may reach maximum values of a hundred or so kilometers and the joining elements may house signal repeaters.
Laying of submarine cables is performed by means of special cable-laying ships which are provided with cable guiding members, usually pulleys with a suitable diameter (of about 3 m for the laying of optical cables and about 6 m for the laying of energy supply cables) so as to allow laying of the cable in water without damaging it. The cable, during laying, must withstand a tractional force which may reach (depending on the weight of the cable itself and the laying depth) several tens of tonnes (for optical cables, the tractional force may vary between 4 and 35 tonnes, depending on the type of cable). When the joining element passes over the guiding member, a situation arises where the joining element contacts the guiding member only in the region of a central portion thereof, while its ends remain at a distance from the above mentioned surface which depends on the curvature of the surface itself and the dimensions of the joining element. Consequently, the cable portion leaving the joining element is not supported by the guiding member surface and, being subjected to a high tractional force directed transversely with respect to the joining element, is subject to severe bending. If the curvature of the cable at the maximum bending point exceeds a critical value, the cable may be damaged. For example, in the case of an optical fibre cable, breakage of the covering sheath of the cable or damage of the optical fibres may occur.
The submarine cable may be, for example, an optical cable of the SA (Single Armour) type. A typical structure of an SA cable comprises a core containing the optical fibres and several coaxial reinforcing and insulating layers, including a load-bearing steel layer (in order to provide the cable with tensile strength), a copper sheath, an insulating layer of polyethylene and an external steel-wire armouring for mechanical protection. The cable may have, for example, an external diameter of 35.9 mm and a weight of 3 kg/m in the air and 2.5 kg/m in the water.
A submarine cable, with a joining element incorporated, in order to be able to be considered suitable for use, must successfully pass bending tests on pulleys as laid down by international standards. The tests include the passage of the cable provided of the joining element over a pulley of 3 m diameter, in accordance with the following criteria (which simulate the severest conditions which could occur during laying):
a) fifty passages under a normal load, i.e. a NOTS (Nominal Operating Tensile Stress) load which in the case of an SA cable is equal to 180 kN;
b) three passages under a load equal to 90% of the breaking load, i.e. a NTTS (Nominal Transient Tensile Stress) load which in the case of an optical cable of the SA type is equal to 260 kN.
In order to ensure that the cable can withstand these stresses, it may be necessary to apply, to the cable portion connected to the joining element, a device which performs the function of a bend limiting device.
In order to protect submarine cables, or in any case cables which are subject to very high loads, from risks of excessive bending in the region of the joints, numerous types of bend limiting devices have been proposed.
The patent application GB 2142788A, in the name of British Telecommunications, describes a bend limiting device comprising a plurality of annular segments which are adjacent and joined together and which define a cylindrical cavity housing the cable portion connected to the joining element. The annular segments are joined together with play by means of bolts and are movable angularly with respect to one another so as to provide the structure with flexibility. The bend limiting device proposed has a limit radius of curvature which depends on the form and the dimensions of the annular segments and the bolts. This bend limiting device has the drawbacks that it is relatively bulky, heavy and complicated to assemble. Moreover, after a certain period of time spent in sea water, corrosion phenomena may result in permanent seizing of the bend limiting device, thereby rendering it useless in the case of recovery of the cable. A similar operating principle is adopted for the bend limiting device according to the patent EP 226188 in the name of Società Cavi Pirelli S.p.A.
The patent U.S. Pat. No. 4,367,967, in the name of Preformed Line Marine Inc., describes a device for relieving the bending forces acting on a cable, comprising a plurality of sleeve elements which are rigid and coaxial with each other and connected together by means of elastomeric material. The adjacent sleeve elements have co-operating surfaces which interfere with one another so as to limit the curvature of the device. More in detail, each sleeve element comprises two longitudinal portions with different diameters. The smaller-diameter portion of each element extends with play inside the larger-diameter portion of the adjacent element. Flexing of the device is made possible, within predefined limits, by the abovementioned play between overlapping portions until the portions themselves interfere with each other. Such a device is described in the patent U.S. Pat. No. 4,396,797 in the name of Horiba Ltd, in which sleeve elements of the type similar to that described above are separated axially from one another by means of flexible rings. Also in the case of the bend limiters according to the U.S. Pat. No. 4,367,967 and U.S. Pat. No. 4,396,797, the high functional interdependence of the sleeve elements, in view of the high stresses transmitted by the cable during bending, may give rise to problems of permanent seizing of the structure.
Further types of bend limiting devices are proposed by the patent application GB 2289577, in the name of Northern Telecom Limited, and the patent U.S. Pat. No. 4,739,801, in the name of Tysubakimoto Chain Co., which both describe bend limiters of the modular type. In particular, the patent application GB 2289577 describes a bend limiter comprising a plurality of adjacent segments in which each segment terminates in a spherical or semi-spherical portion designed to engage in an adjacent segment. The patent U.S. Pat. No. 4,739,801 describes a bend limiter comprising a plurality of segments terminating on both sides in semi-spherical portions and joined together by means of further segments having spherical cavities which are designed to receive the semi-spherical portions.
The abovementioned bend limiting devices suitable for use with high loads are semi-rigid structures which are designed to allow bending of the cable up to a predefined limit whereupon, once reached, they lock together owing to mechanical interference between adjacent rigid elements, thus preventing excessive bending. When the cable passes over a guiding member, the action of these bend limiting devices is independent of the supporting action of the guiding member itself.
With the present invention, the Applicants propose a device which, in the case of submarine cables or, more generally, in the case of cables which must pass over a rigid guiding member of the pulley type (or other similar type), is able to achieve a reduction in bending of the cable in the region of the joining element itself, by exploiting, differently from that which occurs in the known techniques, the supporting action provided by the guiding member itself. The present invention proposes, in particular, exploiting the supporting action of the guiding member by means of a bend limiting device which performs mainly the function of a spacer between the cable and the guiding member, thus allowing the cable to transmit onto the surface of the guiding member itself the tensions applied onto it and progressively move towards this surface, preventing the occurrence of excessive local bending.
The device according to the present invention is designed to be applied to the cable in the region of a rigid joining element and comprises a plurality of rigid elements housed in a flexible body made of polymeric material. Unlike that which occurs in the limiting devices of the known type, these rigid elements are operationally independent of one another. For the purposes of the present invention, the term xe2x80x9crigid elementsxe2x80x9d is understood as referring to elements which have a compressive strength and a flexural strength such as not to undergo significant deformations when the device is subjected to loads of the order of magnitude of those normally acting on the cables considered here. Moreover, for the purposes of the present invention, the expression xe2x80x9cmechanically independent elementsxe2x80x9d is understood as meaning elements which are devoid of mechanical connections and formed so as to avoid, under any operating conditions of the device, interference between their parts.
In the device according to the present invention, the rigid elements are annular elements which perform, independently of one another, the function of spacers between the cable and the surface of the guiding member and are kept in position by the polymeric material of the flexible body.
The particular form of the rigid elements and the characteristics of the polymeric material provide the device according to the invention with a flexural strength such as to allow the device itself to perform its function of bend limiter device also in the absence of an external supporting surface, provided that the loads acting on the cable are not particularly high (for example during cable handling operations prior to laying for storage and maintenance of the joining element). The Applicants have moreover found that the presence of through-holes in the inserts allows the strength of the polymeric body to be increased in the region of the inserts, preventing in particular separation between the polymeric material and the inserts themselves occurring when the device is subject to flexural and/or tractional stresses.
The device proposed is designed with dimensions depending on various parameters, including the type and dimensions of the cable, the dimensions of the guiding member, the dimensions of the joining element, the value of the tractional force to which the cable must be subjected and the maximum value of curvature to which the cable may be subjected.
The bend limiting device according to the present invention is simple and economical to manufacture, has small dimensions and is relatively lightweight. Moreover, this device, being devoid of mechanically co-operating rigid parts, is free from risks of permanent seizure.
According to a first aspect thereof, the present invention relates to a system for the transmission of signals or energy, comprising a cable for the transmission of signals or energy, at least one joining element for joining different portions of said cable, and a bend limiting device applied to said cable in the region of said joining element; said device comprising a flexible tubular body housing a portion of said cable, and at least two rigid elements housed in said body and defining spacing elements in the radial direction for said cable portion, said rigid elements having different external diameters and being arranged in a decreasing order of diameter away from said joining element, said system being characterized in that said rigid elements are mechanically independent of one another.
According to a further aspect thereof, the present invention relates to a device for limiting the bending of a cable, comprising a body made of polymeric material and designed to house a portion of said cable connected to a joining element and comprising moreover at least two rigid elements housed in said body and defining spacing elements in the radial direction for said cable portion, said device being characterized by the fact that said rigid elements are mechanically independent of one another.
In particular, said rigid elements have a substantially annular shape and different diameters and are arranged in decreasing order of diameter away from said joining element. Moreover, said body has a tubular shape and incorporates said rigid elements.
Preferably, said rigid elements have through-holes housing said polymeric material. Moreover, each of said rigid elements has an axis and comprises a radially internal portion and a radially external portion, said radially internal portion having axial dimensions which are greater than said radially external portion. In particular, said radially internal portion has axial dimensions ranging between a first value equal to the diameter of said cable and a second value equal to twice the diameter of said cable.
Preferably, said radially internal portion defines a sleeve portion and said radially external portion defines a substantially annular flange portion. Said sleeve portion has preferably a radially internal surface defining a support surface for said cable, said support surface comprising a central surface portion with a minimum diameter and end surface portions with a diameter increasing from said central surface portion.
Preferably, said body houses a rigid connection element for connection to said joining element, said connection element having through-holes housing said polymeric material.
Preferably, said polymeric material is an elastomeric material, more preferably a polyurethane rubber, an ethylene-propylene copolymer or a silicone rubber.
Preferably, said rigid elements are made of metallic material. Alternatively, said rigid elements are made of thermoplastic material or thermosetting material.